In another bowl, whip the butter and the sugar with a hand-held mixer until fluffy, about 5 minutes. Beat in the egg and vanilla until just combined. Slowly beat in the dry ingredients in 2 additions, mixing just until incorporated.

Scoop the dough into 1-inch balls with a cookie or ice cream scoop and roll in sugar. Place about 2 inches apart on the prepared baking sheets. Press a thumbprint into the center of each ball, about 1/2-inch deep. Fill each indentation with about 3/4 teaspoon jam.

Bake cookies until the edges are golden, about 15 minutes. (For even color, rotate the pans from top to bottom about halfway through baking.) Cool cookies on the baking sheets. Serve.